Lighting fixtures utilizing fluorescent tubes are well known and find widespread usage in all kinds of residential, industrial, commercial and institutional environments, and can be categorized as direct or indirect. "Direct lighting" fixtures, which heavily predominate, direct light downwardly from above, often passing through some form of diffuser to reduce glare and soften harshness, but otherwise travelling directly from the fluorescent tubes to the targeted area in a single straight line path. "Indirect lighting" fixtures are made and/or arranged to shield room occupants from the direct light of the tube, while illuminating a targeted area with reflected light; thus instead of reaching the targeted area via a single straight line path, the path becomes folded as the light is redirected by one or more reflective surfaces. Indirect lighting fixtures can be further subdivided into two main categories:
(1) external reflection fixtures which rely on white or other light-colored room surfaces, typically white ceilings, as the principal reflecting surface. Usually the main housing is a compact elongated enclosed rectangular box containing associated wiring, connectors, ballasts and/or transformers, etc. The fixture is mounted with the fluorescent tube on top, directing light upwardly to the ceiling region, while some form of baffle shields room occupants from direct view of the tube(s). Such fixtures may utilize the small amount of auxiliary reflection available from the top side of the housing beneath the tube by making it white or otherwise reflective. Additional auxiliary reflection may be picked up by the addition of side wings, which can also serve as shielding baffles;
(2) internal reflection indirect fixtures wherein substantially all reflection occurs within the fixture by means of a built-in shaped reflector, preferably made with a specially curved mirror surface, constructed and arranged to project light in a pattern of required uniformity in a targeted area with the fixture installed at a designated room location. A fixture of this category can be beneficially recessed in a selected wall or ceiling location to accomplish the desired illumination. The internal reflection category is advantageous over the external reflection category in eliminating dependency on the efficiency and color of the ceiling, and in enabling control of the illumination pattern.
When utilized in ceiling arrays, light fixtures are generally arranged in an array that yields a grid pattern of (ideally) square or rectangular illumination zones that overlap in a manner to provide acceptably uniform overall illumination. It is also important to suppress the escape of light from the fixtures at high angles from vertical that tend to cause annoying glare from the fixtures as perceived by room occupants. The compromise dictated by these two conflicting requirements becomes more critical with lower ceiling height, e.g. 8 or 10 ft.
A useful definition for the illumination boundary of a fixture is the half-power point where the intensity, i.e. candlepower, falls off to half as related to regions of maximum intensity, since at a reference horizontal targeted plane, e.g. floor or desktops, ideal overlapping from an adjacent fixture at the boundary would restore the intensity to full value. Thus the pattern of light divergence from the fixture can be defined as two angles from vertical, one, referred to as "0.degree." or "parallel", being taken at half power in a first vertical plane through the main lamp axis, the other, "90.degree." or "perpendicular" being taken at half power in a second vertical plane perpendicular to the lamp axis.
To optimally satisfy the two conflicting requirements described above for low ceilings, it has been found that the half power angle should be made at least about 45.degree. to minimize the number of fixtures required but should be held below about 55.degree. to minimize glare.